Cmos-compatible germanium tunable laser

ABSTRACT

A semiconductor light emitter device, comprising a substrate, an active layer made of Germanium, which is configured to emit light under application of an operating voltage to the semiconductor light emitter device, wherein a gap is arranged on the substrate, which extends between two bridgeposts laterally spaced from each other, the active layer is arranged on the bridgeposts and bridges the gap, and wherein the semiconductor light emitter device comprises a stressor layer, which induces a tensile strain in the active layer above the gap.

TECHNICAL BACKGROUND

An efficient electrically-pumped light emitter integrated in thestandard CMOS technology has been so far the Holy Grail of themonolithic electronic-photonic integration. As a matter of fact, rapidadvances in Si photonics over the last decade have enabled massproduction of higher-functionality and lower-cost photonic integratedcircuits, in which all the active and passive components such aswaveguides, couplers, modulators, photodetectors, etc., except for thelight source, can be fabricated side by side with both digital andanalog circuitry in a silicon CMOS foundry. The obvious main advantagesof a fully integrated light source are cost reduction, yield, easing ofthe packaging, link-budget improvement and a consequent powerconsumption reduction.

Among the different pathways leading to the on-chip integration of thelight source, epitaxial lasers on silicon comprising active regionsbased on III-V or SiGe heterostructures have attracted a wide interest.In particular, Ge heteroepitaxial layers on Si are very promising sincekey photonic components for this material system, including high speeddetectors and modulators, have been successfully integrated in standardCMOS process flow. Thus, Ge is now a “fab”-compatible material producedby means of fully qualified production processes and is considered oneof the most promising materials for “more than Moore” devicedevelopment.

An optically pumped Ge-on-Si laser demonstrating continuous-wave (cw)operation at room temperature has already been fabricated, as reportedby J. Liu et al. Optics Letters 35, 679 (2010). In this approach,although Ge has an indirect band gap, the authors exploit a tensilestrain in the Ge layer caused by a difference in thermal expansioncoefficients between the Ge layer itself and the silicon substrate andaccumulated during the fabrication process. Strain ε is defined as

${ɛ = \frac{a - a_{0}}{a_{0}}},$

wherein a denotes the lattice constant of the strained lattice and a₀denotes the lattice constant of the unstrained lattice of the solidstate material under consideration. Moderate strain of the order of 0.2%was reported sufficient to reduce the energy difference between the

and L valleys in the conduction band of the energy band structure ofGermanium. Free electrons, incorporated through n-type doping, can fillup the low-lying L valley so that injected electrons do not thermalizeat L but at

, being thus available for radiative recombination through a directtransition.

The net gain is determined by the competition between this optical gainenhancement and the optical loss from additional free carrierabsorption. In optically pumped bulk-Ge lasers, using a very high levelof doping of 8×10¹⁹ cm⁻³, a net optical gain as high as 500 cm⁻¹ couldbe achieved, as reported by X. Sun et al. IEEE J. of Sel. Topics inQuantum Electr. 16, 124 (2010).

The presence of a thermal tensile strain of 0.2% can double such avalue. However the maximum reported gain for highly doped, thermaltensile strained structures is 50 cm⁻¹, owing to the difficulties in toreal high- and ultra-high n-doping in Ge due to donor solubility, dopantactivation, and material processing. Therefore P. H. Lim et al., OpticsExpress 17, 16358 (2009), proposed to reduce the need of high-dopinglevels by externally increasing the tensile strain in the Ge epi-layerusing micromechanical engineering.

SUMMARY OF THE INVENTION

A first aspect of the present invention a Ge-based light emitterstructure that is CMOS compatible.

Another aspect of the present invention is a CMOS compatible processenabling the fabrication of a Ge-based LED/Laser.

According to the first aspect of the present invention, a semiconductorlight emitter device is provided, comprising

-   -   a substrate;    -   an active layer made of Germanium, which is configured to emit        light under application of an operating voltage to the        semiconductor light emitter device; wherein    -   a gap is provided, which extends between two bridgeposts        arranged on the substrate and laterally spaced from each other;    -   the active layer is arranged on the bridgeposts and bridges the        gap, and wherein    -   the semiconductor light emitter device comprises a stressor        layer, which induces a tensile strain in the active layer above        the gap.

The semiconductor light emitter device of the present invention providesan innovative concept for a CMOS-compatible, electrically drivenGe-based LED or laser structure.

The device is based on the recognition that the Ge active layer may beshaped as an air bridge that is under the influence of a stressor layer.The action of the stressor layer is to induce a tensile stress in the Geactive layer.

As a consequence of the tensile strain, the electronic energy bands aremodified in a way that the radiative recombination of electron-holepairs is enhanced in comparison to the Ge bulk case. Moreover, theenergy-band gap is shrunk in comparison with that of unstrained Ge. As aconsequence, the light emission efficiency of the material is increased,with the emission occurring at a wavelength increasing with the amountof tensile strain.

As will be disclosed in the context of the description of preferredembodiments, the light emitter device of the present invention forms anadvantageous device platform that allows adding tuning elements, whichenable a tuning of the emitted wavelength by strain adjustment, eitherfixed via proper selection of the stressor material, or even variableunder operation of the device via an electrical control.

In accordance with a second aspect of the present invention, a processfor fabricating a light emitter comprises

-   -   providing a substrate;    -   fabricating an active layer made of Germanium, which is        configured to emit light under application of an operating        voltage to the semiconductor light emitter device,    -   fabricating a gap on the substrate, which extends between two        bridgeposts laterally spaced from each other; wherein    -   the active layer is fabricated so as to bridge the gap between        the bridgeposts, and wherein    -   a stressor layer is fabricated, which induces a tensile strain        in the active layer above the gap.

In the following, embodiments of the above two aspects of the inventionwill be described. The additional features distinguishing the differentembodiments can be combined to obtain further embodiments.

The following specification first turns to embodiments of the lightemitter device of the first aspect of the invention.

The active layer bridging the gap is preferably arranged as an outerlayer of the bridge in order to allow inducing the tensile strain.Whether the bridge above the gap is bent upward (away from thesubstrate) or downward (towards the substrate) is a matter of processingoptions and photonic design. Both alternatives form respectiveembodiments of the light emitter device of the present invention.

A Ge light emitting diode (LED) or laser diode (LD) in accordance withthe present invention is preferably configured as a lateral p-i-n diodeoperating under forward bias condition. Electrical contacts to the Gemicrobridge structure are preferably arranged laterally with respect tothe active layer, i.e., when the structure is displayed in across-sectional view as in FIG. 1, in areas to the left and right of theactive layer. In those lateral areas heavy p and heavy n-doping ispreferably applied.

The electrical contacts from the side areas may for instance be made ofpoly-Si or a germanide of a metal, such as Co, Ni, Ti. Also W metalcontacts could be used as direct contacts to Ge.

The active layer bridging the gap between the bridgeposts is herein alsoreferred to as a microbridge. The part of the microbridge structure withhighest strain is preferably of intrinsic conductivity and configuredfor efficient light emission by direct recombination.

The spatial separation of the high doping areas and the light emissionarea brings about key advantages. These include a low free-carrierabsorption and a low Auger recombination. Furthermore, due to a loweringof the conduction band minimum and an increase of the valence bandmaximum in energy under tensile strain, electrons as well as holes willdrift to the highly strained intrinsic part and, due to this localconfinement by band gap engineering, be available for efficient directrecombination and light output.

The light output at the desired wavelength can be further enhanced bythe presence of a photonic crystal design in the intrinsic, highlytensile strained part of the Ge microbridge structure.

Preferably, the tensile strain in the active layer is more than 1%, inother embodiments even above 2%. Increasing the tensile strain to thislevel induces a band gap shrinkage and a further shift of the

and L conduction bands, which in turn progressively decrease theirenergy distance, eventually leading to a cross-over toward a“direct-type” gap.

Such a high level of strain allows reaching a net gain value as high as2000 cm⁻¹ with doping levels even below 2×10¹⁹ cm⁻³, for instance as lowas 8×10¹⁸ cm⁻³. This is a much easier-to-achieve donor density. Thispreferred decrease of the requested donor density in the presentembodiment has another beneficial effect. As a matter of fact, below theof 2×10¹⁹ cm⁻³ limit, the gain is expected to increase with theoperating temperature with a gain maximum achieved at 350 K, i.e., at atemperature very close to the on-chip temperature in standard CMOSdevices.

Nonetheless, other, in particular higher doping levels can be used inother embodiments to partially compensate the strain-induced band gapshrinkage to maximize the optical gain at a desired emission wavelength.

In some embodiments, the substrate is a silicon-on-insulator substratehaving a top silicon layer on an intermediate insulator layer, which isarranged on a carrier substrate, and wherein the gap reaches through thetop silicon layer. The carrier substrate may be made of any materialthat meets mechanical and electrical requirements of the device and ofthe processing technology to be used in fabrication of the device.Silicon is a suitable carrier in particular in the context of a CMOSfabrication process.

In one embodiment of the invention the static stressor is a dielectricor a dielectric layer stack deposited on the active Ge layer. The stressin the stressor layer induces a tensile strain in to the Ge activelayer.

In one type of embodiments, the stressor layer either comprises orconsists of a material layer deposited immediately on the active layer.In these embodiments, a layer stack comprising the active layer and thestressor layer may forms the air bridge that bridges the gap between thebridgeposts. The material choice of the stressor layer depends on thedesired tensile strain that is to be induced. The dielectric layer canfor instance comprise SiN, SiON, or Silicon-rich SiON. One skilled inthe art can find other suitable materials for the stressor layer basedon these examples. An example of a suitable material for inducing atensile stress of more than 1% is silicon nitride. The strain level maybe adjusted by an appropriate choice of the thickness of the stressorlayer, its composition and deposition conditions.

In another embodiment, the stressor layer comprises an electricallytunable layer, which is configured to induce the tensile strain or atensile strain component in the active layer upon electrical actuationof the tuning layer. Suitable tuning layers include a bimorph, that is,a layer stack of a piezoelectric material and a metal. The piezoelectricmaterial is in one embodiment in contact with the active layer.

An actuation, that is, application of an electric field to the bimorph,causes the piezoelectric layer to extend and the metal to contract,which will induce a bow in the active layer that creates strain. Thisstrain adds to the strain created by the stressor layer.

In one embodiment, the voltage-dependent piezoelectric response of thebimorph is transformed into a strain tuning range of the active layer,thus creating to a tuning range of the wavelength of emitted light inresponse to a voltage range of a tuning voltage applied to the bimorph.In this embodiment the tensile strain can be micromechanically tuned andthe density of active dopants can be selected in a wide range ofconcentrations.

Examples of suitable piezoelectrics are ZnO and/or AlN, Differentembodiments have the piezoelectric layer in polycrystalline orepitaxially grown form.

In another embodiment, the stressor layer consists of the tuning layer.

The stressor layer is in an alternative embodiment arranged remotelyfrom the active layer. For instance, the stressor layer may be arrangedwith only indirect mechanical contact to the active layer, in particularbelow the active layer.

A preferred embodiment of the process for fabricating a semiconductorlight emitter device in accordance with the second aspect of theinvention comprises

-   -   providing the substrate comprises providing a        silicon-on-insulator substrate having a top silicon layer on an        intermediate insulator layer, which is arranged on a carrier        substrate, wherein subsequently    -   the active layer and the stressor layer are deposited on the top        silicon layer and laterally etched to define a device layer        stack;    -   a lateral access for an etchant to the top silicon layer and to        the insulator layer is fabricated;    -   the top silicon layer and the insulator layer underneath the        device layer stack are partially removed by applying the        etchant, thus forming the gap.

This CMOS-compatible processing enables a cost-efficient fabrication ofthe light emitter device in large volumes in the context of standardprocessing techniques.

In the following, additional embodiments will be described withreference to the enclosed Figures.

FIG. 1 shows a schematic cross sectional view of an embodiment of alight emitter device.

FIG. 2 shows a schematic perspective view of another embodiment of atuneable light emitter device

FIGS. 3A to 3H show schematic top and cross sectional views of a lightemitter device during different stages of an embodiment of a fabricatingprocess;

FIG. 1 shows a schematic cross sectional view of an embodiment of alight emitter device 100. The graphical representation of FIG. 1 issimplified in that only a lateral section of the device is shown.However, since the device is symmetrical, the parts not shown do notcontain structural features differing from those shown in the Figure.The symmetry is of mirror type, and the position of the mirror plane M,which extends perpendicularly to the cross-sectional plane of FIG. 1, isindicated at the right edge of FIG. 1. Horizontal dotted lines at theleft and right edges of the layer structure are provided to more clearlyshow the respective position and thickness of the individual layers inlayer structure of the light emitter device 100. A furthersimplification of the graphical representation in FIG. 1 is that onlystructural elements are shown, which are essential to understand thestructure of the present embodiment. In particular, no contactstructures are shown.

In the embodiment of FIG. 1, a Si substrate 102 is patterned to exhibita bridgepost 104 formed by a surface section of the substrate 102, and agap 106 formed by a shallow trench in the substrate 102. An active layer106 made of Ge is arranged on the bridgepost 104. An active layer 108 iscovered by a dielectric stressor layer 110, made for instance of SiN.The active layer 108 is made of intrinsic Germanium, and highly doped p-and n-type layers (not shown) are arranged to the left and right of theactive layer, respectively, so as to form a lateral p-i-n structure.

The effect of the stressor layer 110 is to stress and bend theunderlying active layer 108. In this way the electronic energy bands ofGe in the active layer 108 are modified in a way that the radiativerecombination of electron-hole pairs is enhanced respect to the Ge bulkcase, as described above in more detail. Moreover the energy-band gap ofGe is shrunk. As a consequence the light efficiency of the active layer108 is increased, with the emission occurring at a wavelength increasingwith the amount of tensile strain set.

FIG. 2 shows a schematic perspective view of another embodiment of atuneable light emitter device. Reference labels used in FIG. 2correspond to those used in FIG. 1 for corresponding structuralelements, except for the first digit, which is a “2” in FIG. 2 and a “1”in FIG. 1.

The embodiment of FIG. 2 is based on the same structural principle asthe embodiment of FIG. 1. In FIG. 2, the active layer 208 is shown tohave three sections 208.1, 208.2, and 208.3. The active layer sections208.1 and 208.3 are arranged on respective bridgeposts (not shown), andthe active layer section 208.2 spans the gap 206 between the activelayer 208 and the substrate 202 only a surface section of substrate 202in the region of the gap 206 is indicated schematically in FIG. 2.

On top of the active layer 208, the stressor layer 210 is arranged, in asimilar manner as in the embodiment of FIG. 1. However, in the presentembodiment, the stressor layer is made of a piezoelectric in order toallow a tuning of the stress exerted by stressor layer 210 on the activelayer 208. To this end, a contact layer 212 is deposited on a section ofthe stressor layer 210 and allows an application of a tuning voltage.The contact layer is made of a metal and has some resilience toaccommodate the amount of motion in the microbridge structure created bythe piezoelectric effect under application of a tuning voltage to thestressor layer 210 via the contact layer 212.

The influence of the piezoelectric effect, if a tuning voltage isapplied, the stress in the active layer increases or reduces to someextent in comparison with the absence of a tuning voltage. This way, theband gap of Ge can be influenced and thus the wavelength of the emittedlight under application of an operating voltage across the p-i-nstructure can be tuned.

In another variant that is not shown, a second electrically actuatedstressor (bimorph layer) can be deposited on top of a steady stressorlayer (e.g., SiN). The purpose of this second stressor is much like thatof the embodiment of FIG. 2, namely, to modulate the strain in the Gelayer via an external bias. In this way the user can tune the emittedlight frequency over a band centred around the zero-field emission.Enclosed in a suitable photonic structure (e.g. external cavity), thismaterial can be the active layer in a tunable NIR/MIR Laser.

FIGS. 3A to 3H show schematic cross-sectional and top views of a lightemitter device during different stages of an embodiment of a fabricatingprocess. The cross sectional views are shown in the respective upperpart of the Figures, and the op view, are shown in the respective lowerpart of the Figures.

The present process is CMOS-compatible. It is performed starting from anSOI substrate 302 (FIG. 3A), having a carrier layer 302.1, for instancemade of silicon, an insulator layer 302.2, for instance made of silicondioxide, and a silicon top layer 302.3.

On the SOI substrate 302, an intrinsic Germanium layer 308.1 (FIG. 3B)and a doped Germanium layer 308.2 (FIG. 3C) are deposited, which laterform an active layer structure 308. On top, a stressor layer 310 isdeposited (FIG. 3D).

Subsequently, patterning starts with the stressor layer, which isselectively removed, leaving only a stripe-shaped section (FIG. 3E). Anetching step performed in the context this processing step does notattack the underlying active layer structure 308. This is only patternedin a subsequent processing step, to the same stripe shape, as shown inFIG. 3F. This step is performed selectively and does not attack thepreviously patterned stressor layer or the underlying silicon top layer302.3 of the SOI substrate 302.

Subsequently, the top substrate layer 302.3 is patterned by fabricationa shallow trench opening, as shown in FIG. 3G. The cross-sectional viewof FIG. 3G is in a plane indicated by a dotted line in the correspondingtop view underneath. For that reason, the active layer and the stressorlayer are shown in a dashed contour only. Finally, an underetch isperformed removing a section of the insulator layer 302.2. At the end ofthis process, the layer structure of the active Ge layer 308.1, thedoped Ge layer 308.2 and the stressor layer is bent due to the tensilestress exerted by the stressor layer, and due to the gap formation.

1. A semiconductor light emitter device, comprising a substrate; anactive layer made of Germanium, which is configured to emit light underapplication of an operating voltage to the semiconductor light emitterdevice, wherein a gap is arranged on the substrate, which extendsbetween two bridgeposts laterally spaced from each other; the activelayer is arranged on the bridgeposts and bridges the gap, and whereinthe semiconductor light emitter device comprises a stressor layer, whichinduces a tensile strain in the active layer above the gap.
 2. The lightemitter device of claim 1, wherein the substrate is asilicon-on-insulator substrate having a top silicon layer on anintermediate insulator layer, which is arranged on a carrier substrate,and wherein the gap reaches through the top silicon layer.
 3. The lightemitter device of claim 1, wherein the stressor layer either comprisesor consists of a material layer deposited immediately on the activelayer.
 4. The light emitter device of claim 3, wherein the stressorlayer either comprises or consists of a silicon nitride layer.
 5. Thelight emitter device of claim 1, wherein the stressor layer comprises anelectrically actuatable tuning layer, which is configured to induce thetensile strain or a tensile strain component in the active layer uponelectrical actuation of the tuning layer.
 6. The light emitter device ofclaim 5, wherein the tuning layer is configured to induce the tensilestrain or tensile strain component in the active layer that is tuneableover a limited range of strain values in dependence on an amount of anactuation voltage.
 7. The light emitter device of claim 1, wherein thestressor layer is arranged below the active layer, that is, closer tothe carrier substrate than the active layer.
 8. The light emitter deviceof claim 2, wherein the active layer comprises an n-doped Ge layerhaving a donor concentration below 2×10^(19 cm) ⁻³.
 9. The light emitterdevice of claim 1, forming a lateral p-i-n diode, wherein the activelayer forms the intrinsic semiconductor layer of the p-i-n diode.
 10. Amethod for fabricating a light emitter device, comprising, providing asubstrate; fabricating an active layer made of Germanium, which isconfigured to emit light under application of an operating voltage tothe semiconductor light emitter device, fabricating a gap on thesubstrate, which extends between two bridgeposts laterally spaced fromeach other; wherein the active layer is fabricated so as to bridge thegap between the bridgeposts, and wherein a stressor layer is fabricated,which induces a bow in the active layer that is indicative of a tensilestrain present in the active layer above the gap.
 11. The method ofclaim 10, wherein providing the substrate comprises providing asilicon-on-insulator substrate having a top silicon layer on anintermediate insulator layer, which is arranged on a carrier substrate,wherein subsequently the active layer and the stressor layer aredeposited on the top silicon layer and laterally etched to define adevice layer stack; a lateral access for an etchant to the top siliconlayer and to the insulator layer is fabricated; the top silicon layerand the insulator layer underneath the device layer stack are partiallyremoved by applying the etchant, thus forming the gap.